Mobile devices are ubiquitous at every corner of the urban life. People carry their mobile phones or fitness tracking devices everywhere they go. Many services have been developed based on where a user is located. For example, an advertisement service may provide advertisements to a mobile device based on whether a user is located close to a particular merchant. In some use cases, when a location includes multiple zones (e.g., a store or a walkway in a shopping mall), it may be desirable to know in which zone a user's mobile device is located.